1. Field of the Invention
The present invention relates to conjugated diene polymers and conjugated diene copolymer blocks, and a process for preparing them. The present invention particularly relates to butadiene and styrenic block copolymers and a process for preparing them.
2. Background of the Art
The preparation of conjugated diene polymers, particularly in the form of block copolymers based upon more than one monomer, is well known. In a synthetic process, an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block polymer so grown is living until terminated.
Butadiene and styrene are often used to prepare such polymers. The most common of these are styrene and butadiene diblock copolymers and triblock copolymers having a butadiene based midblock with styrene endblocks. In some instances, coupling agents can also be used to couple still living polymers to prepare higher molecular weight polymers. In the production of the most common triblock copolymers, after the butadiene midblock is prepared, additional styrene is then added to the molecule to form a styrene endblock. These polymers are often referred to as SBS copolymers. Such polymers are often sold with retained unsaturation, but hydrogenated forms wherein some or all of the unsaturation has been eliminated are also available.
Microstructure control of conjugated diene polymers or conjugated diene copolymer blocks within block copolymers can be important because a controlled degree of branching in the polymer can be desirable. In the case of butadiene, if the polybutadiene or polybutadiene block is all straight chain, such as in 1,4-polybutadiene, then when the polymer is hydrogenated it will have a polyethylene-like structure and have the corresponding crystallinity associated with such a structure. If the polybutadiene or polybutadiene block is branched, such as in 1,2-polybutadiene, the crystallinity will be lessened or eliminated, which can introduce thermoplastic and elastomeric properties into the polymer.
Microstructure control may be used to prepare a block copolymer based upon only a single monomer. The blocks in such a copolymer differ only in the nature of the addition of the monomer species. This type of block copolymer has been prepared from diene monomers using anionic polymerization techniques. If the diene monomer is butadiene, the first block might be prepared without the addition of a microstructure control agent which would afford a segment which is predominately the result of 1,4-addition of butadiene. A microstructure control agent could be added to the living polymer at that point and additional butadiene added. Polymerization of a second block of butadiene would ensue which would be higher in 1,2-addition of butadiene. The 1,2-addition of butadiene is often referred to as vinyl addition as the product introduces a vinyl group pendant to the polymer backbone. A two-block copolymer prepared in this way would have blocks derived from butadiene that differ only in vinyl content.
The glass transition temperature (Tg) of conjugated diene polymers is also affected by microstructure control. The more 1,4-polybutadiene structure present, the lower the resulting Tg of the butadiene segments and the lower the service temperature of the polymer. Microstructure can also affect melt compatibility with polyolefins such as polypropylene and polyethylene.
Particularly for SBS thermoplastics, it is desirable that the microstructure of the block copolymer includes at least some degree of branching or vinyl content in the polybutadiene midblocks. The resulting block copolymers can then have the desired thermoplastic and elastomeric properties, such as lower glass transition temperature, usually designated using the symbol Tg, and hardness. This is known in the art and it is disclosed in references such as U.S. Pat. No. 5,795,944 to Graafland, et al. Therein, it is disclosed to use a microstructure control agent in two or more doses to closely control the vinyl content of a polymer, in some cases even varying the vinyl content within different regions within the midblock. Block copolymers having specific microstructures can also be prepared by coupling as is shown in EP 0 387 947 B1 to Van der Huizen, et al.
It would be desirable in the art of preparing conjugated diene polymers and conjugated diene copolymer blocks to prepare the polymers or copolymer blocks with both higher and lower amounts of branching as compared to conventional microstructure control technology.